1. Field of the Invention
The present invention relates to a fuel injection system in which a solenoid valve is provided in the middle of the fuel supply path leading to the compression space of a injection pump main body so that fuel injection to a engine is controlled by opening and closing a fuel supply path with this solenoid valve.
2. Description of the Related Art
Fuel injection pumps of this type in the known art include, for example, an invention disclosed in Japanese Unexamined Patent Publication 1-151751 in which a pilot injection is implemented prior to the main injection. In order to achieve the main injection and a pilot injection preceding the main injection, a solenoid valve is driven twice during the compression feed process.
To explain this, reference is made to the characteristics chart shown in FIG. 11 in this publication. The drive pulse supplied to the solenoid valve consists of a main pulse with a long interval and a sub pulse with a short interval. The small injection required to induce ignition of the fuel is achieved by supplying this sub pulse. Generally, the required quantity for a pilot injection is very small; approximately 3-5 mm.sup.3 /st per unit stroke of the plunger. During the restricting process of the fuel supply path, i.e. when the sub pulse restricts the fuel supply path, the solenoid valve is fully lifted (full valve lift), reaching the required quantity before it is seated. To counter this problem, it is designed in such a manner that the supply period of the sub pulse is shortened so that the solenoid valve returns before it is fully lifted.
Also, as shown in FIG. 1 in this publication, a structure in which a piezo element 47, for example, for opening and closing the injection nozzle 49 is provided in addition to the solenoid valve 7, a path 85 which communicates with the path 83 for fuel supply is provided in the housing 11 against which the plunger 23 slides and path 87 for fuel escape that communicates between this path 85 and a pressurized region 17 within a specific cam angle range is formed in the plunger 23.
However, since the solenoid valve receives the instruction to close and be reset before it is fully lifted, the degree of lift is not stable. Because of this, even though a pilot injection is employed to achieve good injection, the overall result is not good because the injection quantity fluctuates and is not stable. Moreover, with the pilot injection adjusted simply with the momentary lift of the solenoid, as in the prior art, when the rotation of the engine speeds up and the main injection quantity increases, the pilot injection also increases. Since, if the pilot injection is large, there may arise adverse effects such as an increase in smoke and the like, the pilot injection ideally should be small regardless of the rate of rotation of the engine.
As for the method in which an electromagnetic valve is driven twice, there is the problem of behavioral instability of the pilot injection and the main injection caused by the response delay of the solenoid valve and the like, resulting in a fluctuation of the injection quantity.
This problem seems to be solvable by applying the aforementioned technology in which a pilot injection is achieved by opening and closing with a piezo element or the like, to start and stop fuel injection promptly. In fact, however, as the plunger does not have a spin stopper, the phase between the plunger and the injection pump main body is not stable. For instance, as shown in FIGS. 12(a) and (b), if the phase between the leak port 52 formed in the injection pump main body 2, and the fuel escape path 43 formed in the plunger 5 changes, the length of the path from the compression space to the leak port and the path resistance also change, resulting in fluctuations in the pilot injection quantity. In this case, if the optimal injection characteristics are achieved in the state as shown in FIG. 12(a) and if the phase changes to the state shown in FIG. 12(b), fuel spill worsens due to the increase in the length of the path and the path resistance. In the worst case, as shown with broken lines in FIG. 8, the pilot injection and the main injection overlap, and stable injection characteristics cannot be achieved.